As is well known from the scientific and technical literature, new asynchronous fast packet switching techniques, also referred to as label-addressed switching techniques, associated with corresponding multiplation and asynchronous transmission techniques, have numerous advantages over traditional synchronous digital switching and transmission techniques. The asynchronous transfer mode is in fact mostly independent of the type and rate of the service considered, and allows a dimensioning of switching apparatus and transmission means based on values near the traffic average to be served. The information generated by the subscriber is, if requested, coded in a digital format and then grouped into packets, whose length is generally predetermined and each packet is provided with a label indicating the call to which it belongs.
Unlike the operations in traditional packet switching, the call is not allotted on each segment a specific circuit or a particular time interval inside a frame, but packets relevant to various calls are forwarded, as they become available, on a single path of the asynchronous multiplex. On this path, which has its own bandwidth dependent on its clock frequency, each call forwards only the packets containing its useful data and then it occupies, at each instant, only the band it needs; this allows a very natural and efficient statistical multiplexing. The switching of a call by a calling subscriber to a called subscriber consists then, at each node, of the switching of a multiplex and of a label-switching on the multiplex.
Technological improvement of transmission means allows HDLC protocol checks to be avoided segment by segment and allows them to be limited to the two connection ends. This considerably improves the system performances, provided switching operation in each node is carried out so as not to introduce in turn excessive delays.
According to presently accepted terminology, the particular packet used in these systems is referred to as a "cell".
The state of the present art on application of these, comprises a number of experimental implementations wherein some renounce label switching, using on all the connection trunks always the same label, i.e. considering the latter as the virtual call indicative on each trunk involved by the call; however, gives rise to label allotting problems, more particularly on networks whose nodes are connected by complex meshes, and requires very large labels, containing parameters for identifying the calling and the called party, and which in turn require long cells to avoid excessive netband losses. There are two disadvantages: long labels are less easy to handle since their hardware processing becomes much complex, while software processing generates an inacceptable time loss; besides long cells degrade the performances of many self-routing switching networks, which are the types of networks which are better suited to asynchronous switching.
In other embodiments label switching is carried out by a centralized processor on the basis of each cell, for each of the multiplexes arriving at the switching node, and this dramatically limits the rates of multiplexes connected or the number of contemporary virtual calls which can be managed; besides multiplexes must be homogeneous, i.e. all must have the same band, and each of them is connected to an input of the connection network. This considerably reduces system flexibility not only as regards its gradual growth, but also a correct and efficient multiplex utilization; in fact, owing to the great band difference the calls can require, if a multiplex is dimensioned so as to allow a reasonable statistical occupancy with wide-band calls, its size will become excessive for narrow band calls, since its occupancy by a reasonable traffic percentage would imply a number of calls difficult to manage, while a sufficient mixing of different-band calls is not always possible or convenient.
In present systems, connections from one point to a plurality of points either are not set-up, or they require special duplicating networks, which can double the size of the connection network, even though such type of traffic is envisaged to be low with respect to total traffic, or even if they require routing towards special-purpose multipoint switching apparatus.
In present implementations the degree of modularity is rather low and each multiplex is connected to a termination of the connection network, which receives only this multiplex. The addition of a new multiplex implies then an enlargement of the connection network, which can be properly exploited only if the multiplex is operable at a high rate; in other words traffic concentration at the switching network input does not exist.